


A Delicate Experiment

by Deburine



Series: Genius Fat Kaito [1]
Category: Vocaloid
Genre: Belly Kink, Body Expansion, Button Popping, F/M, Fluff, Mind Scientist Kaito, Rapid expansion, Student Miku, Teacher-Student Relationship, Weight Gain, fat kink
Language: English
Status: Completed
Published: 2020-08-20
Updated: 2020-08-20
Packaged: 2021-03-06 17:40:41
Rating: Mature
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 4,368
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/26012764
Author URL: https://archiveofourown.org/users/Deburine/pseuds/Deburine
Summary: The Genius chemistry professor Kaito is great with science but bad with people. Hard at work at a "sucrose serum", an accident happens when he's distracted by his student assistant Miku... leading to him accidentally ingesting the experimental compound. Faced with a rapidly fattening body, it's a race against time for Kaito to try and fix the weighty disaster as even more disasters seem to stack up against him!(Mature because of the kink, there's no smut.)
Relationships: Hatsune Miku/Kaito
Series: Genius Fat Kaito [1]
Series URL: https://archiveofourown.org/series/1888036
Comments: 2
Kudos: 14





	A Delicate Experiment

  
  
The university lab was largely empty as Kaito carefully measured the boromine powder as he mixed it into the test tube on his lab station. By this time of the day, most of the classes were done and he often could get the labs to himself for hours of quiet experimentation and scientific research. He normally wouldn't have started right before he had to go teach his own class for the day, but sometime over lunch hour the idea had struck him like a flash of inspiration and he'd spent most of the afternoon scribbling down theoretical formulas and ideas.  
  
The mixture in the test tube was largely pale white, but if his calculations were correct... "The sucrose extract should turn the entire concoction pink..."  
  
He grabbed an eye dropper and stuck it into the container full of syrupy liquid. He squeezed out a few drops into the test tube and watched it change to a lovely rosey color. He clapped his hands for joy. "Success!" he shouted.  
  
"Oh, Sensei, am I interrupting?!"  
  
Kaito turned his head to see his lab student assistant, Hatsune Miku, carrying piles upon piles of papers in her arms. She wore her traditional sky blue sweater over a black plaid skirt, black knee-length stockings, and black loafers. Her red glasses looked about to fall off her face. He quickly rushed over to her before the entire pile hit the floor and grasped them from her hands. "I... I'll... take over this task...  
  
Kaito silently cursed as he saw her upset face while he grabbed the papers. "I just... mean... it's... h-h-h-h-heavy..."  
  
Kaito rested them on his desk in the back of the lab, trying to recover some of his lost composure. "Sensei... is this a new formula?" Miku asked, straightening her red tie, "It's so pretty... like a flower..."  
  
As thoughts of science flew through Kaito's head, he darted back to his station. He grabbed the test tube, pointing to it in triumph. "It's a sucrose stabilization syrum!" he said, "We're always trying to find ways to take care of human caloric intake during situations of food scarcity... but then over lunch, I was contemplating the nature of ice cream and I thought there might be a way to combine a sustainable caloric load into just a drop! Think of all the lives that could be saved! The ventures humans could take if they needn't worry about carrying food!"  
  
Whenver he spoke of science, Kaito's shyness and stutter vanished almost immediately. Maybe because when he spoke of science, he could imagine only himself and the abstract concept instead of himself and another person.  
  
"Oh, so that's what the equation over here on the white board is?!" Miku said, glancing to Kaito's wild scribbles on the whiteboard he'd rolled over next to his lab station, "There's boromine... maldine... oh, this is so simple!"  
  
"Y...y...y... you understand all of it?"  
  
Miku looked embarrassed. "You didn't think I could?" she asked.  
  
He cursed to himself again. "I just.... I mean, I wrote it so... fast... I...."  
  
He tried to shake his nervousness off. "Er... it's not... long before classes... you... have other things, r-r-r-r-right?"  
  
Miku nodded her head. "Okay... those are just the tests from this week... I graded them just like you said."  
  
He nodded his head, his tongue freezing up in futile silence as he leaned his arm on the table. Kaito had a reputation for being a snob, only loving science and himself. He never really knew how to convey that he just never knew how to relate to other people - he wasn't trying to be rude... he wanted their friendship far more than he wanted respect. But nobody ever seemed to understand the world as he saw it. All organic life was a magnificant combination of chemical reactions and science could seek out the answers to its endless mysteries. But trying to convey that never came across as him trying to spread love, but only as trying to show off. So he usually just tried to not talk at all, letting his work speak for him.  
  
But somehow ever since Miku had taken on the position of being his assistant, he'd found his job even harder. Not because the girl herself was lacking in any qualities - to the contrary, she was quick to master new concepts and an invaluable help in his work. She'd just given yet another demonstration of her keen intellect as she translated his rushed scribblings into practical use. Furthermore... she seemed to possess that inscrutable sociability that Kaito himself lacked.  
  
The problem was that more than any person he'd ever met, Miku was literally the hardest person for the brilliant scientist to speak to. Somehow his nervousness always increased around her, making him inevitably say the wrong thing and hurting her feelings. She seemed such a lovely and cheerful girl, that he wanted more than anything to try and express his appreciation of her... but his anxious tongue would end up spewing insults instead.  
  
Had Kaito not been so horribly distracted by his social anxiety, he might have been more careful with his "grand triumph" and adjusted the way he held the test tube so it was _not_ being poured like a fine pink syrup over a small bowl of strawberry ice cream on his lab station...  
  
As Miku left, Kaito rested the tube in its holder and grabbed for his ice cream dish. After so much brain exertion, not to mention the social exertion, he needed the sweet release of a cold treat to calm himself before he gave his next lecture. As he stuck the spoonful of ice cream in his mouth, he noticed the sweetness was far greater than normal. He looked at the dish. "I remember putting some strawberry syrup on it... but not this much..." he murmured, "I'll have to remember that brand though... this is the best I've ever had!"  
  
Within a matter of moments he'd downed the entire bowl - a little faster than normal, but when it tasted this good, he couldn't help himself. Besides, he was in a hurry, it's not like he had time to savor it. He needed to go over the papers before class and...  
  
...suddenly Kaito clutched at his stomach as he felt a massive cramp in his abdomen. His stomach seemed to shiver under his touch. "What... what in the world?" he murmured.  
  
He glanced back at his work station and his eyes went wide as he saw his precious sucrose syrum test tube was more than half empty. _He hadn't even tested it yet._ He had no idea what, if any, the reaction would be on a human subject.  
  
As he felt his stomach cramping again, he realized he was about to find out. As the cramping ceased, Kaito thought perhaps the reaction might wind up mild. Until suddenly his stomach rapidly inflated in front of him, making a soft "fwoomph!" sound as it bloated like rising dough, giving him a noticeable pot belly.  
  
Kaito stared at the orblike flesh straining against his shirt and vest as though it weren't even part of him. As nothing else happened for a few moments, he poked at it with his finger. In spite of how rounded his belly looked, it felt soft instead of pressurized.  
  
"That's odd," he said, "Even given the caloric intake well exceeding a normal nutritional value by several magnitudes... It shouldn't have accumulated in just one place..."  
  
Kaito was cut off as his stomach gurgled and trembled and just as suddenly as before inflated even larger, taking Kaito by surprise as he stumbled from the sudden addition of weight to his now lopsided body. Now his stomach was the size and shape of a beach ball! Whatever was happening, it wasn't stopping! He tried to quickly run the math in his head, trying to guess how many calories he'd ingested due to his own carelessness. The numbers he worked out were... Significantly higher considering the small amount of belly he had...  
  
FWOOMPH!  
  
Kaito's shirt buttons were growing ever more strained under his expanding form - his stomach had doubled in size in seconds. "Ah! The science can wait, I need... a way to reverse this!"  
  
He rushed to his white board and grabbed a marker, scribbling out calculations quickly. He already knew what was in the formula... so it was just a matter of trying to counteract the agents altering his body now...  
  
FWOOMPH!  
  
His belly blew up again, almost throwing him off balance. Now it had gotten large enough that he looked nine months pregnant... with triplets! And he wasn't actually sure how his shirt was still withstanding the growth... "Focus!" he shouted to himself, "I can solve this! This is easy!"  
  
In moments he had another formula perfectly sketched out on the board... fortunately all the components he needed were already on his lab station so he could work quickly and...  
  
"Um, sensei?"  
  
He heard Miku's voice as the door creaked open. Kaito nearly jumped - he couldn't let her see him like _this!_ He quickly dove behind the rolling whiteout board, his belly brushing against it as he hastened to get somewhere where Miku couldn't see him and his swollen stomach...  
  
  
Miku saw the whiteboard shaking on its stand as her teacher stood behind it. "Ah! Miku, w-w-w-w-what is it? Sorry, I'm... in... the middle of a very... important project and..."  
  
She started to step into the room, her feet tapping on the floor, when suddenly Kaito's voice rose. "WAIT! I mean, I can't... be disturbed!"  
  
'I must have really made him mad this time...' Miku thought, stopping her entrance entirely as she pressed her fingers together.  
  
"Are you cancelling your class then?" she asked timidly.  
  
A moment's silence - it was almost as if Kaito had forgotten. Then again, of course a brilliant scientist like himself could get absorbed in a wonderful discovery, to the point of shutting out the world around him until he finished. She looked to the lab station and saw the test tube with his sucrose serum was emptier. No doubt he'd started testing it and he'd had another great breakthrough...  
  
"M...M....Miku..." Kaito said, "You know this material quite well, yes? Most of the class is going to be a review of the test... you graded all of them and... I know you understand it well enough that you could answer questions on it."  
  
"AH! You want me to teach your class!?"  
  
Miku was stunned - was he just shirking his duty or...  
  
"Miku. I trust you. You're smart and you're good with people. I know my students will do wonderful with you taking over today."  
  
Her heart began to race - her teacher... liked her?! He even TRUSTED her!? Her face lit up like a sky full of fireworks. "Sensei, I won't let you down! Thank you!"  
  
As she grabbed the test papers and skipped out of the room... she thought she heard a sound... if she were to describe it, something like "fwoomph!" But why worry about that when she finally had the chance to prove to him how good she was?!  
  
  
As he heard the door slam shut, Kaito stared at the buttons scattered on the floor in front of him. The last round of his belly's expansion had popped off the bottom two on his shirt and his vest buttons were hanging on for dear life. He had to put an end to this quickly before he ruined his clothes entirely. At least once he got rid of the belly fat he could mend his shirt... thank goodness he hadn't let this serum go into _human_ testing...  
  
He rushed to the other side of the board, his belly bouncing around like it were made of jello, and examined his notes and scribbles carefully. He couldn't get this wrong... he grabbed the components he needed, reciting some of them aloud as he did so...  
  
"The purminium compound should counter the excess cellulose... the triturdium will balance the sucrose production..."  
  
He kept eyeing his notes as he hurried to mix everything together...  
  
FWOOMPH!  
  
Kaito's belly slammed and smooshed into the counter, pushing him backwards with the momentum. Kaito gripped the edge for dear life lest he topple over - if he hit the floor, he wasn't entirely sure he'd be strong enough to lift himself back up again with so much weight working against him. Grunting, he lifted his sagging stomach up and rested it on the table. At least now it was large enough that it could be supported by something. He realized he should have locked the door when this madness began, but he admitted to himself he had little time and had been too distracted by his new situation to do so...  
  
"It's ready!" he shouted, staring at the light blue liquid in his test tube, "This should solve everything!"  
  
As Kaito downed the substance, he saw his stomach quivering as though it were about to grow again. He hoped he'd consumed the counter-solution in time...  
  
But fortunately, his stomach did not surge forward yet again. Kaito hesitated to try and imagine how much he weighed with so much fat accumulated in one place. Yet his logical mind was already trying to do the calculations... he continued to watch his stomach, hoping it would shrink at any moment... but then he felt a strange queasy sensation sweep through his entire body...  
  
All of a sudden, Kaito's belly began to lose its rounded shape. At first he hoped it was because he was shrinking, but it simply seemed to be altering to become more of a flabby belly than a stuffed belly. Kaito felt a tightness around his upper arms, where he'd rolled his sleeves up. To his surprise, it appeared that his arms were starting to swell with fat. His shirt seemed to grow tighter and tighter as even his chest swelled up, his pecs turning into moobs. Kaito backed away from the table, his belly overhanging his pants and sagging over his thighs, which were now rounded like two thick bundles of dough and squeezing up against the fabric of his pants. He put a hand to his soft stomach, feeling his chubby fingers sinking in as it continued to grow along with the rest of him. He estimated he was blowing up so fast, he had to weigh five hundred pounds - but he wasn't slowing down in the slightest! One by one the buttons on his shirt and vest flew off in rapid succession, revealing more and more of his belly and chest as his tie hung over bare flesh.  
  
He hadn't _fixed_ anything - he'd made it _worse!_ Kaito took a panicked look at the board. What had he done wrong?!  
  
Then... he saw it... he should have seen it! Smudges in the marker... when he'd rushed to hide from Miku, his belly had rubbed against the board... he'd unintentionally erased part of his formula.  
  
Kaito heaved himself over, erasing the mistaken chemical notations with his pudgy fist and quickly jotting over the mistakes. He stared at his lab station and noticed with dismay one of the chemical compounds he needed was in the supply closet. Kaito pushed the board out of the way, trying to waddle to the small room as his body simply grew heavier and heavier with every passing second. He had limited time to grab it and mix a counter solution that worked... his every footstep lead to the fat on his body jiggling and wobbling. He could feel his thighs rubbing together as he huffed and puffed trying to move so much bulk. He was only a few feet away from the closet door, but as he calculated he'd passed 550 pounds, he realized it may as well have been miles away in his old body. Finally he gripped the door frame for dear life, spying the bottle he needed in the back....  
  
...only to find that the closet doorway was _not_ large enough to accommodate a man of his size. Kaito kept trying to squeeze his way inside, trying to force his body through the narrow opening. So what if he got stuck?! He had to chance it, if he didn't get the compound he needed, his ballooning weight would make it impossible for him to fix his problem. Finally, Kaito backed away from the door frame, swallowing deeply before hurling himself at it, hoping to use his momentum to get inside.  
  
He failed, bouncing off and landing firmly on his enormous backside. He heard a tearing noise and felt a draft and his face turned red as he realized the force had torn open the seat of his pants. Kaito desperately looked for some way to get off his bottom... a table, anything... only to realize he was just too far. He folded his legs up, trying to heave his body back up off the floor... but to Kaito's dismay, he couldn't get his 600 pound body to do little more than wobble.  
  
He heard several more tearing noises and saw his pants splitting up the sides, revealing his fat thighs would no longer be constrained. His shirt seemed headed for the same direction as his massive moobs pressed against it. He felt a release around his stomach and saw his belt slip to the floor as it came undone, his pants button breaking when his heavy stomach dropped on top of it. Kaito could do little but watch himself grow and hope against hope at some point it was going to stop...  
  
After half an hour, Kaito had passed the 800 pound mark and his body was still swelling with every passing moment. He lamented what would happen when he was finally discovered... the people who already thought so little of him would think him a laughingstock, constantly mocking him behind his back. And that was assuming any one of them could actually put the formula together to shrink himself back to a reasonable size! He swallowed as he imagined the school calling out a crane and fire crew to haul his fat body off the property...  
  
Finally the door swung open and he braced himself...  
  
...of _course_ it would be Miku, the unintentional source of all of his problems.  
  
"Sensei, sensei, everyone was thanking me when they left! I answered all their questions, just like you would do and..."  
  
He saw her eyes bug out behind her glasses as she saw him. If Kaito ever thought he was nervous around her before, now she made him feel utterly catatonic, appearing before his student that finally thought the world of him as a gelatinous pile of blubber. "Sensei! What happened?!" she gasped out.  
  
"I... I must have... accidentally... ingested the sucrose serum..." Kaito said, barely able to overcome his shyness to explain his dilemma, "I... I... I wasn't... I couldn't fix it..."  
  
Words failed him to admit to such a failure of genius. He barely looked at her as she quickly locked the lab door behind her and then bolted over to his whiteboard. "Sensei, is this... a counter-formula?!" she said, her eyes darting across his hasty scribbles.  
  
She seemed to be processing the equations in her head... perhaps... he wasn't completely helpless...  
  
"M...M...Miku..." Kaito said, trying so desperately to control his stutter, "The only ingredient I was missing is in the supply closet... I-I-I-I-I can't seem to..."  
  
"Don't worry sensei, I'll fix this in a jiffy!" Miku said, giving him a thumbs up and a smile, "If you wrote this formula, I know it will work!"  
  
Miku darted into the closet, easily clearing the door her obese teacher had failed to cross himself and rushing back out. She seemed to recognize the time element, hastily moving across the table and glancing back at his board. All the while, Kaito was still growing, taking up more and more of the lab as his belly had forced his legs apart, blanketed his thighs, and was starting to grow past his feet. His lab coat was giving out, tears springing up along the sleeves. The last few buttons on his shirt gave out, exposing his entire chest as it sprang apart, his tie flopping down. His shoes felt uncomfortably tight.  
  
But finally Miku finished her work and brought the test tube over to Kaito. "I... I promise this will make you better!" she said, though from the way her hands shook from the way she held the test tube, he could tell how nervous she was. He had to admit his nerves were working against himself as well as he drank the substance hoping that he'd finally fix his problem.  
  
His stomach gurgled and groaned, and for a few awful moments Kaito feared another awful side effect. But finally... his body stopped growing. He felt some disappointment when he realized that was all the two of them accomplished this time as after several minutes, he wasn't any skinnier. But at least he'd stopped his growth, though he estimated by now he weighed around 900 pounds...  
  
Miku let out a sigh of relief. "Well... at least you're not going to get any _bigger,_ " she said.  
  
Kaito owed her for helping him, but he couldn't help but feel humiliated she'd seen him like this. Somehow... it just felt _worse_ that it was _Miku_ who had seen him. He still didn't understand. What _was_ it about Miku that always made him extra nervous and concerned about what she thought of him?  
  
Miku leapt forward and gave her teacher a huge hug, not that her arms reached that far. "I'm... I'm so glad you're safe! I'm so glad that you're..."  
  
Suddenly her face turned beet red and she jumped back. 'Of course...' Kaito thought bitterly, 'It must be repulsive to her to touch me and feel all this... blubber...'  
  
"I'm s-s-s-s-s-sorry you... had to see such... a ... grotesque display..." Kaito said, flushing deeply with embarrassment.  
  
Miku shook her head. "N...no! That's not it! I just... I shouldn't just hug you, sensei... we barely know each other, it was rude of me..."  
  
Well, at least she was kind about lying. "I can't imagine you think that highly of me now..." Kaito lamented, "Even if we figure out how to reverse this... mess..."  
  
He ran a hand sadly along his mountainous belly. "How could someone like you possibly take me seriously anymore?"  
  
"NO!" Miku shouted, "I... I would never think that way about you, sensei! You're... you're the smartest, coolest guy I've ever met! And... and..."  
  
She blushed deeply, as if there was something she was having trouble saying. "I... sensei... even if you weighed two tons... I... would still find you... the handsomest guy I've ever met too..."  
  
 _Handsome?!_ She thought he was... _handsome?_  
 _  
_And suddenly everything clicked in Kaito's head. He understood why Miku could distract him, why he cared so much about her opinions. Because... she was the only person he'd _ever_ cared about.  
  
"You're... also... quite pretty... Miku..."  
  
Miku squealed with glee on hearing the compliment. She dove onto Kaito's stomach, causing him to grunt as she pressed her small weight against his. She crawled up and gave him another hug around his chest. "I... I always thought you... hated me... you thought I was stupid and I always felt like I was because you're so smart and..."  
  
There were tears in her eyes but her beautiful smile illuminated her face. Kaito just pulled Miku into a tight embrace, enjoying the feeling of her pressed up against him. Sure he was gargantuan in size right now, and his thick arms spread along her back, but as she snuggled up to him, he cared a lot less about his current physical dilemma. For a few minutes they just stayed like that, Miku cuddling against Kaito's body, running her soft hands along his chest, Kaito running his puffy hands through her silky pigtails. Finally she crawled over his moobs and gently kissed Kaito, caressing his numerous chins as she did so. The euphoria of human contact overwhelmed him - he'd never imagined anything could light his heart as much as science, but this connection with his dear student did.  
  
Love... was not very scientific.  
  
Finally Miku crawled off of Kaito's body and stood up. "Okay... sensei..."  
  
"Please, just call me Kaito..." he said.  
  
Miku smiled. "Kaito!" she said, as if she loved saying his name, "We have to figure out a formula to get you skinny again or you're not going to be able to walk home tonight!"  
  
She gave his tall belly a soft pat. "I don't mind this of course... but I'm sure you'd like to be able to stand again, right?!"  
  
  
After an hour of student and teacher bouncing ideas off of each other, they finally settled on one last experiment to try on Kaito. He drank one last solution and after a few moments, his whole body seemed to deflate as though he were a training dummy with a hole spilling out sand. They'd run the calculations perfectly to ensure Kaito drank just enough to lose all the weight he'd gained in the last several hours... and fortunately, they were right.  
  
Unfortunately, he'd pretty much ruined his clothing - it was torn and stretched beyond recognition. Fortunately, he'd brought his jogging suit and he quickly escaped to change into something that actually covered him. "Well, I'm not sure how useful all of this will be," he called out from the closet, "But I'm sure somehow I've forwarded the cause of science today! I just never expected to run so many experiments on _myself_ in a single day!"  
  
Miku nodded as she finished scribbling down all the notes on the white out boards in her notebook, as well as notes on where to acquire all of the chemicals. As Kaito exited the closet in his sweatsuit, she closed her notebook quickly lest he notice. She rushed over and hugged him tightly. As he squeezed her back, she realized that as much as she loved hugging him at all, like this... she'd learned today there was one other way she liked him... and it wouldn't be that hard to get _that_ Kaito back either...

**Author's Note:**

> Oh boy, this was my first story where I wrote a Kaito that wasn't the Software Glitch Kaito. I based his appearance on the [Mind Scientist/Genius Module](https://media.tumblr.com/tumblr_m8ewslmZ7N1qfc7tn.jpg), with Miku being the [Deep Sky](http://img2.wikia.nocookie.net/__cb20120810025546/vocaloid/images/7/7f/F-deepsky.png) student uniform. Generally Genius Kaito is portrayed as very cold but at the time I wanted to throw a curveball and have him only _appear_ cold with the truth being that he's actually very kind but so socially awkward that he can't speak to people right.
> 
> This fanfic was the first of many I did where I had a commission or a collaboration involved. The entire endeavor was inspired when my quiet partner, who I dubbed the Ghost Artist, shared a [very funny picture](https://sta.sh/0f4scfiilec) of the Mind Scientist Kaito having messed up an experiment. I'm linking their work because you're about to see a *lot* more of the Ghost Artist in my uploads.
> 
> (NOTE: I used to have artwork in this story by another artist that has left the kink community and asked to have their art removed. I have complied with the request and will not be distributing the art further.)


End file.
